Alkaline electrochemical cells having zinc anodes and manganese dioxide cathodes have become commercially important in recent years. Such cells, particularly when manufactured in the cylindrical configuration are very important sources of portable electrical energy. Alkaline zinc manganese dioxide cells provide substantially more energy vis-a-vis Leclanche cells when used in high current continuous discharge applications. For example, when compared to Leclanche cells of the same size, alkaline zinc-manganese dioxide cells deliver more than twice the usable power in light load conditions, such as the Light Industrial Flashlight (LIF) test, but up to eight times the usable power under heavy drains.
The principal factor for the increased capacity of alkaline-zinc manganese dioxide electrochemical cells is the presence of the alkaline electrolyte. In such cells, at least some of the alkaline electrolyte is mixed with the zinc, usually comprised of a fine powder, to form a gelled anodic mixture. The alkaline electrolytes, which usually comprise concentrated aqueous potassium hydroxide and a minor part of the zinc oxide, provide for more complete electrochemical reactions as compared with the electrolytes used in Leclanche cells.
It is well known that strong alkaline electrolytes such as concentrated potassium hydroxide oxidize, i.e., corrode, zinc upon contact. In fact, as soon as a zinc-alkaline electrochemical cell is closed during the manufacturing process, the oxidation of zinc by the alkaline electrolyte can occur. Therefore, unless steps are taken to impede the oxidation of the zinc, alkaline electrochemical cells may have little or no usable life once they are placed into service. The corrosion of the zinc anodic material by alkaline electrolyte correlates roughly with temperature, i.e., the higher the storage temperature of electrochemical cells having zinc anodes oxidizable by electrolyte, the more dramatic the decrease in cell capacity.
The oxidation of the zinc by the electrolyte can also undermine the structural integrity of small electrochemical cells. A by-product of the oxidation reaction between zinc and the electrolyte is hydrogen gas. If the hydrogen gas evolves at a very rapid rate the cell may explode. However, even if the cell does not explode, the build-up of hydrogen gas can so weaken the seals of the cell that it begins to leak.
Historically, small amounts of mercury, e.g., one part mercury for every seven to twelve parts zinc, have been added to the powdered zinc anodic material of alkaline zinc-manganese dioxide electrochemical cells to control the corrosion of zinc by alkaline electrolyte. Mercury, as it will do with most metals, combines with the zinc to form a zinc/mercury amalgam. It is well known that such zinc/mercury amalgams increase the hydrogen overvoltage, i.e., the voltage level at which hydrogen will be given off by a metallic element in contact with an acidic or an alkaline solution.
As the percentage of mercury in the zinc/mercury amalgam increases, the hydrogen overvoltage increases. Or, to put it another way, as the proportion of mercury in the amalgam increases, the rate at which zinc oxides and evolves hydrogen gas in presence of alkaline electrolyte decreases. Unfortunately, as evidence of the toxicity of mercury to man and other animals, and to the environment in general, becomes more well-known, it is becoming increasingly unacceptable to use mercury to solve the problem of the oxidation of zinc in the presence of alkaline electrolyte of alkaline zinc cells.
The problem of the oxidation of zinc in the presence of alkaline electrolytes in electrochemical cells is well-known and a myriad of solutions to the problem have been suggested. For example, U.S. Pat. No. 4,377,625 to Parsen et al. proposes the use of certain amine-containing chelating agents to suppress hydrogen gas evolution in alkaline (or acidic) galvanic cells. While the Parsen et al. reference stresses the unique nature of its solution to the problem of zinc corrosion, it also catalogs a number of other patents which propose at least partial solutions to the gas evolution problem in alkaline zinc electrochemical cells. Most of the solutions discussed in Parsen et al. provide for the addition of various compounds, often well-known hydrogen gas inhibitors, to the zinc anode. For example, U.S. Pat. No. 2,897,250 to Klopp suggests the use of 8-nitro quinoline and 8-chloro quinoline; U.S. Pat. Nos. 3,285,783, 3,291,645 and 3,291,646 to Gould suggest various nitrogen containing compounds which are comprised of aliphatic hydrocarbon radicals; and U.S. Pat. No. 3,963,520 to Bauer et al. suggests monocarboxylic acid containing at least two ethanolamide radicals. However, neither the Parsen et al. reference or any of the other references cited therein, suggest the use of a class of compounds which provide for a reduction in the amount of hydrogen gas evolving from the anode of alkaline-zinc manganese dioxide cells while simultaneously allowing for a reduction in the amount of mercury used in the amalgamated zinc containing anodes of such cells.
The reduction of the amount of mercury in alkalineo zinc manganese dioxide cells is the focus of U.S. Pat. No. 3,847,669 to Paterniti. In the Paterniti reference, the amount of mercury incorporated into the anode mixture of alkaline zinc manganese dioxide cells can be reduced by adding at least 0.01% by weight zinc of an ethylene oxide polymer to the cell. The solution suggested by Paterniti, however, suggests that the zinc anodes contain up to 8 percent by weight mercury. By today's increasingly rigid environmental standards, it is unacceptable to provide for alkaline electrochemical cells containing eight percent mercury. Therefore, in spite of the zinc anode additive taught in Paterniti, further reductions in the amount of mercury used in alkaline zinc electrochemical cells must be developed.